Panda cub

Ultrasound

Pointing out the fetus

Bai Yun

Yun Zi

Gao Gao

Gao Gao looking away

Medical staff gathered

SAN DIEGO --For weeks, San Diego Zoo staffers knew that Bai Yun, a 20-year-old female panda, might be pregnant. But given that she had had an unsuccessful pregnancy in 2011 -- and because only one other known panda has ever given birth at such an age -- they kept their enthusiasm in check.

But last Friday, based on several factors, including indications from a pair of high-tech tools used to monitor Bai Yun, they issued a birth watch, and today, Bai Yun delivered. No one knows yet the sex of the newborn cub, nor will they for weeks. But who cares? It's a newborn panda.

Panda cub

Ultrasound

Pointing out the fetus

Bai Yun

Yun Zi

Gao Gao

Gao Gao looking away

Medical staff gathered

One of the tools the staff used to monitor Bai Yun's pregnancy was ultrasound. Here, in this ultrasound image, it is possible -- if you know what you're looking for -- to see evidence of the in-utero cub.

Gao Gao looking away

Medical staff gathered

Panda cub

Ultrasound

Pointing out the fetus

Bai Yun

Yun Zi

Gao Gao

Gao Gao looking away

Medical staff gathered

This is Yun Zi, Bai Yun's 3-year-old son, who's still at the San Diego Zoo but may soon be sent back to China under an agreement that says that China has the right to claim any three-year-old pandas born outside the country.